1. Field of the Invention
The present invention relates to a method of fabricating a glass substrate for a disk.
2. Description of the Prior Art
Recently, there has been needed an optical memory element as a high density mass memory element. The optical memory element can be classified into a read only memory, a write once memory and a rewritable memory according to its working form. The optical memory element to be used as the write once memory and rewritable memory uses a substrate for a disk. The substrate for a disk preliminarily comprises on a glass substrate guide tracks for guiding optical beams for recording, reading and erasing information to predetermined positions on the optical memory element, and track addresses for identifying track numbers. The track is divided into a plurality of sectors. In the case where information should be managed, sector addresses and the like are often provided in advance. As shown in FIG. 8, the depth of a guide track portion 1b is not equal to that of a sector (or track) address portion 1a, and the address portion 1a is provided between the guide track portions 1b. The reason is as follows. In the case where the optical beams are tracked by an optical differential method, it is preferred that the depth of the guide track portion approximates .lambda./(8.times.n) and the depth of the address portion approximates .lambda./(4.times.n) (where .lambda. is a wavelength of light to be used and n is a refractive index of a substrate). As shown in FIGS. 9 to 13, there has been proposed a technique for forming the guide track, the sector (or track) address and the like on a glass substrate by a contact printing method and a dry etching method.
Referring to the technique described above, a photomask is used for forming a glass substrate for a disk by the contact printing method and the dry etching method. The photomask has such a shape that a quantity of light radiated onto the guide track portion and sector (or track) address portion on a photoresist film is varied. This technique will be summarized with reference to FIGS. 9 to 13.
As shown in FIG. 9, a positive type photoresist film 2 is provided on a glass substrate 1, and prebaking is then carried out.
As shown in FIG. 10, a photomask 3 having a thin film 4 for a mask is caused to contact the photoresist film 2 through the thin film 4. In this case, the thin film 4 is not provided in a formation area A1 of a sector (or track) address portion 1a. Consequently, a quantity of transmitted light is not reduced. The thin film 4 remains at a thickness of about 10 to 200 .ANG. in a formation area B1 of a guide track portion 1b. Consequently, a quantity of transmitted light in the formation area B1 is smaller than in the formation area A1. Then, ultraviolet rays 5 are radiated to expose the photoresist film 2 provided below the photomask 3 (see FIG. 10). Thereafter, the photomasks 3 and 4 are removed.
As shown in FIG. 11, the photoresist film 2 thus exposed is developed, and postbaking is then carried out. Exposure is fully carried out by strong light in a formation area A2 of the address portion 1a on the photoresist film 2. Consequently, the photoresist film 2 is completely developed so that the glass substrate 1 appears. In a formation area B2 of the guide track portion 1b on the photoresist film 2, the exposure is carried out by weaker light than in the formation area A2. Consequently, development is stopped halfway. As a result, the photoresist film 2 remains by a thickness according to a quantity of transmitted light in the formation area B1 on the thin film 4, so that the glass substrate 1 does not appear. The reference numeral 22a denotes a residual photoresist film.
As shown in FIG. 12, the glass substrate 1 is subjected to dry etching using gas such as CF.sub.4 or CHF.sub.3. The photoresist film 2 shown in FIG. 11 is also etched simultaneously. Consequently, when etching is started, the etching of the glass substrate 1 is advanced in the formation area A2 so that a deep pit 11a is formed on the glass substrate 1. The residual photoresist film 22a is etched in the formation area B2. When the glass substrate 1 appears, the etching of the glass substrate 1 is advanced. Consequently, there is formed a groove 11b which is shallower than the pit 11a in the formation area A2. The reference numeral 22b denotes a photoresist film which remains when the etching is completed.
As shown in FIG. 13, the photoresist film 22b remaining on the glass substrate 1 is removed to form on the glass substrate 1 the address portion 1a as a deep pit 111 and the guide track portion 1b as a shallow groove 112. Thus, there is formed a glass substrate 11 for a disk on which the address portion 1a and guide track portion 1b have different depths.
The depth of the groove 112 forming the guide track portion 1b is defined by the thickness of the photoresist film 22a remaining on the guide track portion 1b, and the etching speeds of the photoresist film 2 including the residual photoresist film 22a and the glass substrate 1 as shown in FIG. 11. The thickness of the photoresist film 22a is defined by a quantity of transmitted light and developing conditions in the guide track portion 1b. Accordingly, it is difficult to uniformly form pits and grooves with good reproducibility.
It is an object of the present invention to provide a method of fabricating a glass substrate for a disk wherein groove-shaped guide track portions and pit-shaped sector (or track) address portions interposed between the guide track portions can uniformly be formed.